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   难度：Medium
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   <h1 class="question_title">
    647. Palindromic Substrings
   </h1>
   <p>
    Given a string, your task is to count how many palindromic substrings in this string.
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   <p>
    The substrings with different start indexes or end indexes are counted as different substrings even they consist of same characters.
   </p>
   <p>
    <b>
     Example 1:
    </b>
   </p>
   <pre>
<b>Input:</b> "abc"
<b>Output:</b> 3
<b>Explanation:</b> Three palindromic strings: "a", "b", "c".
</pre>
   <p>
    &nbsp;
   </p>
   <p>
    <b>
     Example 2:
    </b>
   </p>
   <pre>
<b>Input:</b> "aaa"
<b>Output:</b> 6
<b>Explanation:</b> Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".
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   <p>
    &nbsp;
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   <p>
    <b>
     Note:
    </b>
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   <ol>
    <li>
     The input string length won't exceed 1000.
    </li>
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   <p>
    &nbsp;
   </p>
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   <h1 class="question_title">
    647. 回文子串
   </h1>
   <p>
    给定一个字符串，你的任务是计算这个字符串中有多少个回文子串。
   </p>
   <p>
    具有不同开始位置或结束位置的子串，即使是由相同的字符组成，也会被计为是不同的子串。
   </p>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre>
<strong>输入:</strong> "abc"
<strong>输出:</strong> 3
<strong>解释:</strong> 三个回文子串: "a", "b", "c".
</pre>
   <p>
    <strong>
     示例 2:
    </strong>
   </p>
   <pre>
<strong>输入:</strong> "aaa"
<strong>输出:</strong> 6
<strong>说明:</strong> 6个回文子串: "a", "a", "a", "aa", "aa", "aaa".
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   <p>
    <strong>
     注意:
    </strong>
   </p>
   <ol>
    <li>
     输入的字符串长度不会超过1000。
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